Image forming apparatus with a plurality of primary transfer sections

ABSTRACT

A first mode to form a toner image of single color where a pressure contact release section T changes a primary transfer section  7  to be in a pressure contact state, and a second mode to form an overlapped toner image where the pressure contact device T changes a plurality of the primary transfer sections  7  to be in the pressure contact state can be executed, wherein a speed difference D 1  between an image carrier  1  and an intermediate transfer belt  6  in the first mode is controlled to be greater than a speed difference D 2  between the image carrier  1  and the intermediate transfer belt  6  in the second mode.

This application is based on Japanese Patent Application No. 2008-299237filed on Nov. 25, 2008, in Japanese Patent Office, the entire content ofwhich is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to an image forming apparatus such as acopying machine, a printer and a facsimile machine having a plurality ofimage carriers at a periphery of an intermediate transfer belt.

BACKGROUND

In recent years, with a trend of developing high speed andhighly-functional image forming apparatuses, image forming apparatusesof tandem method using intermediate transfer belts have become availablein the marketplace. The image forming apparatus using the intermediatetransfer belt is usually provided with image forming units configuredwith image carriers to develop different colors of toner, chargingdevices, writing devices, developing devices and cleaning devices at avicinity of the intermediate transfer belt. By the image forming units,a plurality of colors of toner images are transferred and overlappedonto the intermediate transfer belt to form a color image and thencollectively transferred onto a sheet at a transfer position. (Forexample, Patent Document 1: Unexamined Japanese Patent ApplicationPublication No. 2001-201902)

In an image forming apparatus of the tandem method, it is not alwayspreferable that the speed of the intermediate transfer belt is equal tothat of a photoconductive drum, and in the Patent document 1, to preventshedding phenomenon at the time transfer, the difference of the movingspeed between the intermediate transfer belt and the photoconductivedrum is purposely created.

Also, in the image forming apparatus of the tandem method, a monochromemode, in which only the black toner is used, may be carried out. In casethe monochrome mode is carried out with the speed difference, in thephotoconductive drums which do not carry out image forming, there havebeen problems that abrasion was accelerated and scratches were likely tobe created by grazing with the intermediate transfer belt, whereby thelife is shortened. To cope with the above problem, in the image formingapparatus of Patent Document 1, in a single color mode, the moving speedof the photoconductive drums which do not carry out image forming arecontrolled to be equal to the moving speed of the intermediate transferbelt.

An image forming apparatus disclosed in the patent document 2(Unexamined Japanese Patent Application Publication No. 2005-156776),there is provided a primary transfer roller which biases theintermediate transfer belt onto the photoconductive drum. By retractingthe primary transfer roller, the intermediate transfer belt can bedisplaced from a position where the belt is biased onto thephotoconductive drum to a released position. In the monochrome mode, byretracting the primary roller to bias the photoconductive drum which isnot involved in image forming, extension of lives of the photoconductivedrums and developing devices which are not used for image forming isrealized.

-   Patent Document 1: Unexamined Japanese Patent Application    Publication No. 2001-201902-   Patent Document 2: Unexamined Japanese Patent Application    Publication No. 2005-156776

In the image forming apparatus of the tandem method, positional accuracyamong the toner images formed by individual image forming units isimportant since an insufficient positional accuracy causes color shiftamong the toner images.

To ensure the positional accuracy, it is important that rotation of theintermediate transfer belt is controlled with high accuracy, and themoving speed of the photoconductive drum is slower than that of theintermediate transfer belt in some cases for the following reasons:

In case a load of a drive system of the intermediate transfer belt isfluctuated, the rotation speed becomes unstable due to effects ofelastic deformation of the drive system and play of gears engaged. Ifthe belt rotates with a load more than a prescribed load, the aboveeffect can be reduced, thus in order to rotate with high accuracy, it ispreferred to drive rotation of the belt while applying the load morethan the prescribed load.

As a device to apply the load, a brake disposed at the drive system ofthe intermediate transfer belt can be considered. However because of acomplicated mechanism and difficulty of applying the prescribed load fora long period of time, it is not a practical. Whereby it is consideredthat by making the moving speed of the photoconductive drum slower, theload is applied from the photoconductive drum to the intermediatetransfer belt.

However, in the image forming apparatus described in Patent Document 2,in the monochrome mode, number of the photoconductive drums in contactwith the intermediate transfer belt is one fourth of that in the fullcolor mode. Thus there is concerned a problem that the load applied tothe intermediate transfer belt is extremely reduced and an effect ofload torque generated when the sheet passes through the secondarytransfer section becomes large, whereby rotation of the intermediatetransfer belt cannot be controlled with high accuracy.

Also, the image forming apparatus described in Patent Document 1, themoving speed of the photoconductive drum is changed for the monochromemode and for the color mode, however since the speed is changed so as tomatch the moving speed of the photoconductive drum matches to that ofthe intermediate transfer belt, there is concern that the load of theintermediate transfer belt reduces. Also, since there is no mechanism torelease the photoconductive drum, there is a concern that an effect toextend the life is not sufficient.

SUMMARY

The present invention has one aspect to solve the above problems in aconfiguration where the number of the photoconductive drums in contactwith the intermediate transfer belt varies with the modes and an objectof the present invention is to provide an image forming apparatus tocontrol the intermediate transfer belt with high accuracy whileoptimizing color shift performance of the toner image and suppressingthe effect caused by reducing number of the photoconductive drums beingin contact.

The above object can be achieved by the following embodiment:

1. An image forming apparatus, comprising:

a plurality of image carriers on which different colors of toner imagesare formed respectively;

an intermediate transfer belt;

a plurality of primary transfer sections, disposed to respectivelycorrespond to the plurality of the image carriers, to transfer eachcolor of toner images formed on the plurality of the image carriers ontothe intermediate transfer belt for forming an overlapped color tonerimage, wherein the primary transfer sections press the intermediatetransfer belt from a back side so that the intermediate transfer beltcomes in pressure contact with the image carriers so as to createtransfer nips between the intermediate transfer belt and the imagecarriers;

a secondary transfer section to transfer the overlapped color tonerimage formed on the intermediate transfer belt onto a sheet;

a pressure contact release section to drive and switch the plurality ofthe primary transfer sections between a pressure contact state where theintermediate transfer belt is in pressure contact with the image carrierand a release state where the intermediate transfer belt is releasedfrom the image carrier;

a plurality of drive motors to drive rotation of the image carriers andthe intermediate transfer belt independently and respectively, capableof varying each moving speed thereof; and

a control section to control the pressure contact release sections andthe drive motors,

wherein a moving speed difference is a moving speed Vb of theintermediate transfer belt minus a moving speed Vd of the image carrier(where Vb>Vd), and the control section controls the pressure contactrelease section so as to execute a first mode to form a toner imagewherein at least one primary transfer section is in the pressure contactstate through the pressure contact release section, and a second mode toform the toner images wherein greater number of the primary transfersections than that in the released state are in the pressure contactstate, and controls the drive motors in accordance with the modes sothat the moving speed difference in the first mode is greater than thatin the second mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a relevant portion of an image formingapparatus 100.

FIG. 2 is a control block diagram of an image forming apparatus relatedto the present embodiment.

FIG. 3 shows a magnified view of a periphery of an intermediate transferbelt 6 in a state of a second mode (full color mode).

FIG. 4 shows a magnified view of a periphery of an intermediate transferbelt 6 in a state of a first mode (monochrome color mode).

FIG. 5 is a schematic diagram showing a relation between a load (weight)and a deformation volume.

FIG. 6 is a schematic diagram showing a relation between a moving speeddifference D and color shift.

FIG. 7 is a control flow which a control section C11 of an image formingapparatus executes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described with reference to theembodiments without being limited the embodiments thereof.

<Image Forming Apparatus>

An image forming apparatus related to the present embodiment will bedescribed based on FIG. 1. FIG. 1 is a view showing relevant portions ofan image forming apparatus 100.

The image forming apparatus 100 is so called a tandem method imageforming apparatus configured with a plurality of sets of image formingdevices 10Y, 10M, 10C and 10K, an intermediate transfer belt 6 in ashape of a belt, a sheet feeding device 20 and a fixing device 30.

At an upper portion of the image forming apparatus 100, a scanner 110 isdisposed. A document placed on a document table is scanned through anoptical system of a document image scanning exposure device of thescanner 110 and read by a line image sensor. An analogue signal havingbeen subject to photoelectric conversion through the line image sensoris inputted to an exposure section 3 after analogue processing, A/Dconversion, shading correction and image compression processing havebeen carried out.

Meanwhile, in the present specification, the elements are denotedcollectively by reference symbols having no alphabetic suffix andelements of individual colors are denoted by reference symbols havingsuffixes i.e. Y (yellow), M (magenta), C (cyan) and K (black).

Each of an image forming device 10Y to form a yellow (Y) color image, animage forming device 10M to form a magenta (M) color image, an imageforming device 10C to form a cyan (C) color image, and an image formingdevice 10K to form a black (K) color image, is provided with a chargingelectrode 2, an exposure section 3, a developing section 4 and acleaning section 5 at a periphery of a photoconductive drum 1 in a shapeof a drum representing a image carrier (in FIG. 1 reference symbols forM, C, and K are omitted).

The photoconductive drum 1 is, for example, composed of an organicphotoconductive body which is configured by forming a photoconductivelayer made of a resin having an organic photoconductive substance at anouter circumference of a metal base in the shape of the drum, anddisposed in the way that the rotation axis of the photoconductive drum 1is perpendicular to a conveyance direction of the sheet S (in FIG. 1, adirection perpendicular to the page surface).

The developing device 4 includes binary developer composed of toner andcarrier having small particle diameter of different colors i.e. yellow(Y), magenta (M), cyan (C) and black (K).

The intermediate transfer belt 6 in a shape of a belt is supportedrotatably by a plurality of rollers. The intermediate transfer belt 6 isan endless belt having a volume resistivity of 10⁶ to 10¹² Ω·cm and is,for example, a semi-conductive seamless belt having a thickness of 0.04to 0.10 mm where a conductive material is dispersed in engineeringplastics such as modified polyimide, thermal curing polyimide, ethylenetetrafluoroethylene copolymer, polyvinylidene-fluoride and nylon alloy.

Toner images of individual colors formed on the photoconductive drum 1by the image forming devices 10Y, 10M, 10C and 10K are successivelytransferred onto the intermediate transfer belt 6 (primary transfer)through primary rollers 7Y, 7M, 7C and 7K (hereinafter collectivelycalled primary rollers 7) to serve as a primary transfer section so asto form a combined color image. On the other hand, after image transfer,residual toner on the photoconductive drums 1Y, 1M, 1C and 1K is clearedby cleaning section 5 of each color.

The primary transfer rollers 7 press the intermediate transfer belt 6onto the photoconductive drums 1 from a rear surface side of theintermediate transfer belt 6. As a result, a transfer nip is formedbetween the intermediate transfer belt 6 and the photoconductive drum 1.

The sheet S stored in a sheet storing section (tray) 21 of the sheetfeeding device 20 is fed through a first sheet feeding section 22, andconveyed to a secondary transfer roller 9 to serve as a “secondarytransfer section” via sheet feeding rollers 23, 24, 25A, and 25B and aregistration roller (secondary sheet feeding section) 26, then the colorimage is transferred onto the sheet S (secondary transfer).

Since three-tiered sheet storing sections 21 disposed in a verticaldirection in parallel at a lower portion of the image forming apparatushave the substantially the same configuration, they are denoted by thesame reference symbols. The sheet feeding device 20 includes the sheetstoring sections 21 and sheet feeding sections 22.

The sheet S on which the color image has been transferred is grasped bythe fixing device 30. By applying heat and pressure, the color tonerimage (or toner image) on the sheet S is fixed on the sheet S. Then thesheet S on which the color toner image (or toner image) has been fixedis grasped and conveyed by a conveyance roller pair 37 and ejectedthrough a sheet ejection roller 27 disposed at an ejection sheetconveyance path and then placed on a sheet ejection tray 90 outside theapparatus.

On the other hand, after the color image is transferred onto the sheet Sthrough the secondary transfer roller 9, a cleaning section 69 clearsthe residual toner on the intermediate transfer belt 6 from which thesheet S has been released by curvature.

In case both surfaces of the sheet S are to be printed, after fixing theimage formed on the first surface of the sheet S, the sheet S branchesfrom the ejection sheet conveyance path via a branching plate 29 andgoes into a both surfaces conveyance path 28, then the sheet S flipsupside down, after that the sheet S is conveyed from a sheet feedingroller 25B. On the second surface of the sheet S, an image of each coloris formed through the each of image forming deices 10Y, 10M, 10C and10K, whereby images are formed on both the surfaces of the sheet S. Thenthe sheet S is subject to the pressure heat fixing process through thefixing device 30 and ejected outside the apparatus through the sheetejection rollers 27.

FIG. 2 is a control block diagram of the image forming apparatus relatedto the present embodiment. In FIG. 2, relevant portions necessary todescribe operation of the present embodiment and their peripheries aremainly shown. Other known portions as the image forming apparatus areomitted. Also, to prevent duplication of description, common portionsare denoted by the same symbols which will substitute for furtherdescriptions.

In FIG. 2, the control section C11 is to control entire operation of theimage forming apparatus provided with a CPU, a ROM and a RAM. In theROM, various kinds of programs are stored and a program downloaded tothe RAM is executed by the CPU. In the ROM of the control section C11,various kinds of tables to be described, such as control tables forrevolution of the drive motors are stored.

The control section C11 is provided with a speed control section, whichcontrols and varies the moving speed of the intermediate belt 6 and thephotoconductive drum 1 by controlling the revolution speed of the drivemotor M1 to drive and rotate the intermediate transfer belt 6, and thedrive motors M2 _(Y), M2 _(M), M2 _(C) and M2 _(K) to rotatephotoconductive drums 1 respectively.

A symbol M3 denotes a drive motor to operate the pressure contactrelease section T so as to contact the primary transfer roller 7 ontothe intermediate transfer belt 6 with pressure and release the roller.

A print controller C14 having a network I/F receives a print job from anexternal terminal such as an external PC connected via a network. Theprint job received is converted by a prescribed page-descriptionlanguage into image data having a data form which is capable of formingthe image in the image forming section 10, and temporally stored in thecontrol section C11 along with control data included in the print job.

<Pressure Contact and Release>

FIG. 3 and FIG. 4 are magnified views of a periphery of the intermediatetransfer belt 6. The intermediate transfer belt 6 is supported byrollers 61 and 64, a tension roller 62 and a drive roller 63. Thetension roller 62 biases the intermediate transfer belt 6 in a Zdirection shown by an arrow in the figure to prevent the belt fromslackness.

A symbol T denotes the pressure contact release section configured witha base T1, a pin T2, a guide T3 and a spring T4. A rotation axis of theprimary transfer roller 7 is supported by the base T1. The base T1slides and moves in side the guide T3 in a direction A shown by an arrowby a pressure of the spring T4 so as to press the primary transferroller 7 onto the photoconductive drum 1. The pin T2 restricts thetravel of the bass T1.

Symbols M1, M2 _(Y-K) and M3 denote drive motors. The drive motor M1drives the drive roller 63 to rotate the intermediate transfer belt 6.The drive motors M2 _(Y), M2 _(M), M2 _(C) and M2 _(K) (hereinaftercollectively called simply drive motors M2) rotate the photoconductivedrums 1 respectively. Meanwhile, the drive motors M2 can vary rotationspeed individually so as to change the moving speed of thephotoconductive drums 1. The drive motor M3 operates the pressurecontact release sections T to switch between a contact pressure statewhere the primary transfer rollers 7 press the intermediate transferbelt 6 onto the photoconductive drums 1 from the back side and areleased state where the primary transfer rollers 7 do not press.

FIG. 3 shows a state of a second mode (full color mode). In the secondmode, the primary transfer rollers 7Y, 7M, 7C and 7K are in the pressurecontact state where the full color image can be formed.

FIG. 4 shows a state of the first mode (monochrome mode). In the firstmode, only the primary transfer roller 7K is in the pressure contactstate by operation of the pressure contact release section T, and otherprimary transfer rollers 7Y, 7M and 7C are in the released state whereinthe axes of the transfer rollers 7 have shifted to an arrow B direction.

<Setting of Moving Speeds of Intermediate Transfer Belt 6 andPhotoconductive Drum 1>

Here, setting of the moving speed of the intermediate transfer belt 6and the photoconductive drums 1 will be described. Incidentally, in whatfollows, the moving speed of the intermediate transfer belt 6 is Vb, themoving speed of the photoconductive drum 1 is Vd (or Vd_(Y), Vd_(M),Vd_(C), and Vd_(K)), and the difference of both the speeds is D=Vb−Vd.Also, the speed differences D in the first mode (monochrome mode), andthe second mode (full color mode) are respectively denoted by addingsuffixes such as the speed differences D1 and D2.

(1) In the tandem method, it is preferred that the speed of theintermediate transfer belt 6 is a standard since a longitudinalmagnification (magnification of sub-scanning direction) has to beadjusted on the sheet.

(2) In respect to rotate control of the intermediate transfer belt 6,both an average value and variation of the moving speed of theintermediate transfer belt 6 have to be controlled with high accuracy.The former causes a problem of magnification and the latter causesuneven pitch in the sub-scanning direction and color shift.(3) In order to control rotation drive in a high accuracy, it ispreferred to continuously apply a load more than a prescribed load ontoa drive system of the intermediate transfer belt 6. Because elasticdeformation is caused by applying the load onto the drive system of theintermediate transfer belt 6, and the deformation amount varies withfluctuation of the load. Also, when the load is light, a fluctuation ofthe deformation amount is large however, if the load is greater than theprescribed load, the fluctuation of the deformation amount tends to besaturated. FIG. 5 is a schematic diagram showing a relation between theload and the deformation amount. In the system shown by the diagram, thefluctuation of the deformation amount reduces when the load more thanthe prescribed value Fx is applied, thus rotation drive becomes stable.(4) As a device to apply the load onto the drive system of theintermediate transfer belt 6, the photoconductive drum in pressurecontact (through bias of the primary roller 7) with the intermediatetransfer belt 6 is preferred to be used. By setting the moving speed Vdof the photoconductive drum 1 lower than the moving speed Vb of theintermediate transfer belt 6 (Vb>Vd), the load can be applied onto theintermediate transfer belt 6 in rotation. Incidentally, it is possibleto apply the load onto the intermediate transfer belt 6 by providing adedicated brake system, however the configuration becomes complicatedsince a position to which the load is applied has to be a vicinity ofthe intermediate transfer belt 6 and applying a stable load for a longtime of period is difficult, thus the brake system is not practical.(5) By lowering the moving speed Vd of the photoconductive drum further,the speed difference D between the intermediate transfer belt 6 and thephotoconductive drum 1 becomes large, on the other hand since thephotoconductive drum 1 is driven while being pulled by the intermediatetransfer belt 6, the load of the photoconductive drum reduces. In thedrive system of the photoconductive drum 1, if the load is excessivelylight (for example, no load or negative load) controllability ofrotation drive is deteriorated and problems such as color shift anduneven pitch occur. Therefore, the problem occurs even if the movingspeed difference D is excessively large.(6) Also, though it is not significant, it is known that there is atendency that color shift is remedied as the moving speed difference Dreduces.

This is because a conveyance force, created by an electro staticadhesion force due to an applied voltage and a frictional force via abias force of the primary transfer roller, is operating between thephotoconductive drum 1 and the intermediate transfer belt 6, however theconveyance force fluctuates with unevenness of surface condition of theintermediate transfer belt 6 and an amount of the toner image existingbetween the photoconductive drum 1 and the intermediate transfer belt 6,i.e. a printing duty. It is considered that since slippage between theintermediate transfer belt 6 and the photoconductive drum 1 having themoving speed difference D occurs or does not occur depending upon theaforesaid fluctuation of the conveyance force, color shift is magnifiedlocally at an area where slippage occurs.

(7) For the above background, an optimum value of the moving speeddifference D is preferred to be set as small as possible in the rangewhere the relation of Vb>Vd is maintained, taking account thefluctuation of the speed in rotation, namely the rage where the movingspeed Vb of the intermediate transfer belt 6 is not smaller than themoving speed Vd of the photoconductive drum 1 (the rage where therelation of speed is not reversed).

FIG. 6 is a schematic diagram showing a relation between the movingspeed difference D and color shift. In an area A shown in FIG. 6, sincea magnitude relation between both the sides are reversed (Vb<Vd), theintermediate transfer belt 6 rotates while being pushed by thephotoconductive drum 1 in the rotation direction and the fluctuation ofthe speed of the drive system of the intermediate transfer belt 6 due todeformation is magnified and then the color shift is magnified. Furtherif the moving speed of the intermediate transfer belt 6 becomes too slowcompared to that of the photoconductive drum 1, gears and couplings ofthe drive system of the intermediate transfer belt 6 are unsettled (thegears and couplings move freely by being pushed forward), thuspositional accuracy is deteriorated. On the other hand, if the movingspeed difference D between both sides is too large as an area B shown inFIG. 6, as described in the item (5), the load of the drive system ofthe photoconductive drum 1 becomes too light and the color shift ismagnified.

An area C in FIG. 6 is a preferable area. As described in item (7), theoptimum value is the moving speed difference Dx which is a smaller valuein the range where the relation of Vb>Vd is maintained, namely in therange where the above speed relation is not reversed. Incidentally, theoptimum value of the moving speed difference Dx varies with variation ofthe moving speeds of the intermediate transfer belt 6 and thephotoconductive drum 1. For example, given that the a set value of themoving speed Vb of the intermediate transfer belt 6 is 400 mm/sec, theoptimum value of the moving speed Vd of the photoconductive drum 1 is399.6 mm/sec and the moving speed difference Dx is 0.4 mm/sec. In theabove case, a difference ratio is approximately 0.1%. Incidentally, themoving speeds Vd_(Y) Vd_(M) Vd_(C) and Vd_(K) of the photoconductivedrums 1Y to 1K are set at the same speed.

<Control Flow>

Next, a control flow will be described. FIG. 7 is the control flowexecuted by the control section C11 of the image forming apparatus.

In Step S11, whether the current job mode is the first mode (monochrome)or the second mode (full color) is judged with reference to control dataof the print job to be executed.

If it is judged to be the second mode, the speed difference (Vb−Vd)between the intermediate transfer belt 6 and the photoconductive drum 1is set to be D2 in a subsequent Step S12. In the former example, themoving speed difference D2 is 0.4 mm/sec (the moving speed Vb is 40mm/sec, the moving speed Vd is 399.6, the moving speed difference D2 is0.4 mm/sec and the difference ratio is 0.1%).

In Step S13, by operating the pressure contact release section T, allthe primary transfer rollers 7 of Y, M, C and K colors are changed to bein the pressure contact state. In this case, as mention in the forgoing,the intermediate transfer belt 6 received the load from thephotoconductive drums 1Y, 1M, 1C and 1K.

On the other hand, if it is judged to be the first mode in Step 11, thespeed difference (Vb−Vd) between the intermediate transfer belt 6 andthe photoconductive drum 1 is set to be D1 in a subsequent Step S21.This means that by lowing the moving speed Vd of the photoconductivedrum 1 in the first mode than that of the second mode, the moving speeddifference D1 in the first mode becomes greater than the speeddifference D2 in the second mode. In the above example, the speeddifference D1 is 2.0 mm/sec (the moving speed Vd is changed from 399.6mm to 398.0 mm/sec and the moving speed Vb is remained at 400 mm/sec.The difference ratio is 0.5%). The reason will be described later.Incidentally, the moving speed difference can be magnified by making themoving speed of the intermediate transfer belt 6 in the first modefaster than that in the second mode. In the above case, since a changeof a longitudinal magnification ratio tends to occur, a lateralmagnification ration (change of writing clock frequency) has to bechanged at the same time.

Subsequently, in Step S14, printing job is executed to form an imageuntil the printing job is completed (Step S15).

In Step S16, an end process is executed. In the end process, all theprimary rollers 7 are changed to be the release state and drive motorsM1 and M2 halt to end.

Here, the reason that the moving speed D1 in the first mode (monochrome)is made greater that the moving speed D2 in the second mode (full colormode) will be described. In the present embodiment, as described in theforegoing <Setting of moving speed>, the load is applied to theintermediate transfer belt 6 by lowering the moving speed Vd of thephotoconductive drum 1 than the moving speed Vb of the intermediatetransfer belt 6. Also, as described in FIG. 3 and FIG. 4, it isconfigured so that the primary transfer rollers 7 can be switchedbetween the pressure contact state and the released state by operatingpresser contact release section T.

As the above configuration, in the second mode (full color), the fourphotoconductive drums 1Y, 1M, 1C and 1K apply load onto the intermediatetransfer belt 6. On the other hand, in the first mode (monochrome), onlythe primary transfer roller 7K is in the pressure contact state. Theload applied to the intermediate transfer belt 6 is given by onephotoconductive drum 1K. Namely, in the first mode, the number of thephotoconductive drums in contact with the intermediate transfer belt 6is one fourth of that in the second mode, thus the load onto theintermediate transfer belt 6 reduces, and as a result the intermediatetransfer belt 6 cannot be controlled to rotate with high accuracy.

To cope with the above problem, as the control flow in FIG. 7 shows, inthe first mode, by further increasing the moving speed D1, compared tothat in the second mode, the load applied by the photoconductive drum 1Konto the intermediate transfer belt 6 increases compared to the secondmode, and decreasing of the load due to the number of thephotoconductive drums 1 in contact is one fourth can be compensated.Incidentally, the load applied to the photoconductive drum 1K is reducedby pulling of the intermediate transfer belt 6, resulting indeterioration of controllability of the rotation drive of thephotoconductive drum 1. However, in the first mode (monochrome), tonerimages do not have to be overlapped inherently, color shift is notnecessary to be considered and only irregular pitch to be considered. Ingeneral, since a tolerance for irregular pitch is wider than that ofcolor shift, it cannot be a serious problem. Incidentally, the movingspeed Vb of the intermediate transfer belt 6 and the moving speed Vd ofthe photoconductive drum 1 are stored in a memory section (notillustrated) respectively in respect to the modes and read out inaccordance with the modes.

In the present embodiment, in a configuration where the number of thephotoconductive drums in contact with the intermediate transfer beltdiffers between the color mode and the monochrome mode, the intermediatetransfer belt can be controlled with a high degree of accuracy whilemaintaining optimum color shift performance in a color mode andsuppressing an effect of change of number of the photoconductive drums.

According to the present embodiment, in a configuration where the numberof the photoconductive drums varies with the modes, it becomes possibleto provide an image forming apparatus to control the intermediatetransfer belt with high accuracy while optimizing color shiftperformance of the toner image and suppressing the effect caused byreducing number of the photoconductive drums in contact.

1. An image forming apparatus, comprising: a plurality of image carrierson which different colors of toner images are formed respectively; anintermediate transfer belt; a plurality of primary transfer sections,disposed respectively to correspond to the plurality of the imagecarriers, to transfer each color of toner images formed on the pluralityof the image carriers onto the intermediate transfer belt for forming anoverlapped color toner image, wherein the primary transfer sectionspress the intermediate transfer belt from a back side so that theintermediate transfer belt comes in pressure contact with the imagecarriers so as to create transfer nips between the intermediate transferbelt and the image carriers; a secondary transfer section to transferthe overlapped color toner image formed on the intermediate transferbelt onto a sheet; a pressure contact release section to drive andswitch the plurality of the primary transfer sections between a pressurecontact state where the intermediate transfer belt is in pressurecontact with the image carrier and a release state where theintermediate transfer belt is released from the image carrier; aplurality of drive motors to drive rotation of the image carriers andthe intermediate transfer belt independently and respectively, capableof varying each moving speed thereof; and a control section to controlthe pressure contact release sections and the drive motors, wherein amoving speed difference is a moving speed Vb of the intermediatetransfer belt minus a moving speed Vd of the image carrier, where Vb>Vd,and the control section controls the pressure contact release sectionsso as to execute a first mode to form a toner image wherein at least oneprimary transfer section is in the pressure contact state, and a secondmode to form the toner images wherein a greater number of the primarytransfer sections than that in the first mode are in the pressurecontact state, and the control section controls the drive motors inaccordance with the modes so that the moving speed difference of theimage carrier(s) of the primary transfer sections that are in thepressure contact state in the first mode is greater than that in thesecond mode.
 2. The image forming apparatus of claim 1, wherein thecontrol section controls a first drive motor so that the moving speed ofthe image carrier in pressure contact with the intermediate transferbelt in the first mode is slower than that in the second mode.
 3. Theimage forming apparatus of claim 2 wherein the control section controlsa second drive motor so that the moving speed of the intermediatetransfer belt in the first mode is faster than that in the second mode.4. The image forming apparatus of claim 1, wherein the moving speed Vbof the intermediate transfer belt and the moving speed Vd of the imagecarrier are stored in a memory section in accordance with the modes. 5.The image forming apparatus of claim 1, wherein a longitudinalmagnification ratio of the color toner image is changed in accordancewith a level of change of the moving speed of the intermediate transferbelt.